Surfactant formulations for foam flooding

ABSTRACT

A foam generating surfactant formulation includes a betaine, an alpha-olefin sulfonate and a nanofluid. The betaine is preferably a cocamidopropyl betaine or laurel betaine. The alpha-olefin sulfonate is preferably an anionic surfactant having between 8 and 18 carbon atoms per molecule. The nanofluid is preferably an oil-in-water nanofluid that includes an emulsifying surfactant, a solvent, a co-solvent and water. The addition of the nanofluid increases the thermal stability and salt resistance of the foam generating surfactant.

RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/624,337 filed Apr. 15, 2012 entitled “Microemulsion for Carbon Dioxide Foam Flooding,” and U.S. Provisional Patent Application Ser. No. 61/733,872 filed Dec. 5, 2012 entitled “Microemulsion for Foam Flooding,” the disclosures of which are herein incorporated by reference.

FIELD OF THE INVENTION

The present invention generally relates to the production of petroleum and more particularly to compositions and processes for improving the recovery of petroleum from a subterranean geological formation.

BACKGROUND OF THE INVENTION

For many years, petroleum has been recovered from subterranean reservoirs through the use of drilled wells and production equipment. During the production of desirable hydrocarbons, such as crude oil and natural gas, a number of other naturally occurring substances may also be encountered within the subterranean environment.

Although supercritical carbon dioxide (CO₂) flooding is a widely used method in tertiary oil recovery, the method presents many challenges, such as inefficient gas utilization, poor sweep efficiency and low oil recovery due to viscous fingering and gravity segregation. One recent development is the application of CO₂ foam in order to reduce the CO₂ mobility, especially in high permeability zones of the reservoir. In the past, CO₂ foam has been produced using surfactant mixtures prepared through the combination of betaines and alpha-olefin sulfonates (AOS). The efficiency of these prior art CO₂ foam efforts often decreases sharply during flooding as a result of contact with crude oil, retention of surfactants on the geologic formation, high salinity in formation water, a lack of reservoir pressure necessary to keep the CO₂ as a supercritical fluid and high reservoir temperatures.

There is, therefore, a need for the development of inventive surfactant formulations which have better tolerance to these factors. It is to these and other objectives that the present invention is directed.

SUMMARY OF THE INVENTION

In preferred embodiments, the present invention includes a foam generating surfactant formulation that includes a betaine, an alpha-olefin sulfonate and a nanofluid. The betaine is preferably a cocamidopropyl betaine or laurel betaine. The alpha-olefin sulfonate is preferably an anionic surfactant having between 8 and 18 carbon atoms per molecule. The nanofluid is preferably an oil-in-water nanofluid that includes an emulsifying surfactant, a solvent, a co-solvent and water. The addition of the nanofluid increases the thermal stability and salt resistance of the foam generating surfactant.

In another aspect, preferred embodiments include suitable methods of using the foam generating surfactant to produce a treatment foam in a well. The method begins with the step of mixing together a betaine, an alpha-olefin sulfonate and a nanofluid to form a foam generating surfactant formulation. The method continues with the step of pumping the foam generating surfactant formulation into the well. The process continues by forcing gas into the well to contact the foam generating surfactant formulation to produce the treatment foam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a graph showing the results of a laboratory test of a preferred embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to a surfactant formulation optimized for use in connection with a foam flooding tertiary recovery method. In preferred embodiments, the surfactant formulation includes three components: (1) an alpha olefin sulfonate (AOS) surfactant, (2) a betaine surfactant and (3) a suitable nanofluid. The addition of a nanofluid to the AOS and betaine surfactants mitigates adsorption and provides additional salt and oil tolerance beyond that of the individual AOS and betaine components. This presents a significant improvement over prior art foam systems.

Betaine is used as a foam booster and a stable CO₂ foamer in fracturing at high temperatures. Suitable betaines include cocamidopropyl betaine and laurel betaine. Suitable cocamidopropyl betaines are commercially available from Rhodia under the Mackam OK50 trademark. Laurel betaine is particularly preferred because it is stable at high temperature and low pH and can generate CO2 foam.

The AOS component is used to maintain and stabilize the foam. The AOS component is preferably an anionic surfactant, shorter chain alpha olefin sulfonate (e.g., C8-C12). In lower salinity environments, it may be possible to substitute the shorter chain alpha olefin sulfonate with a longer chain AOS (e.g., C12-18), which may exhibit improved foam generation properties. One preferred AOS component is Stepan's Stephantan AS 1246. It is believed that the combination of different types of surfactants synergistically exhibits better foaming properties than those of individual components.

If the betaine and AOS components are combined with a nanofluid, the nanofluid is preferably an oil-in-water nanofluid that includes an internal oil phase distributed within an external water phase through use of one or more surfactants. The internal oil phase is preferably a solvent selected from the class of solvents referred to as citrus terpenes, with d-limonene being a particularly preferred solvent. Suitable nanofluid surfactants include surfactants and surfactant mixtures having a hydrophile-lipophile balance (HLB) of between 8-20. Particularly preferred surfactants include mixtures of ethoxylated castor oils and ethoxylated alcohols. In a more particularly preferred embodiment, the selected surfactant or surfactant mixture has an HLB value of between 8 and 18.

In a particularly preferred embodiment, the surfactant component is an ethoxylated alcohol. In a more preferred embodiment, the surfactant component is an ethoxylated C8-C18 alcohol. In a still more preferred embodiment, the surfactant component is an ethoxylated C8-C18 alcohol with 5-20 moles of ethylene oxide (EO). In a particularly preferred embodiment, the surfactant component is an ethoxylated vegetable oil. In a more preferred embodiment, the surfactant component is an ethoxylated castor oil. In a still more preferred embodiment, the surfactant component is an ethoxylated castor oil with 25-45 moles of EO. U.S. Pat. No. 7,380,606 issued to Pursley, et. al on Jun. 3, 2008 entitled “Composition and Process for Well Cleaning,” which is incorporated herein by reference, discloses several nanofluid formulations that may function as the nanofluid component of the CO₂ foam surfactant formulation. In certain applications, it may be desirable for the nanofluid component to include a mixture of different surfactants and surfactant packages.

The effectiveness of the preferred embodiments was evaluated against control foam generating systems using laboratory testing procedures. As a control, a foam generating surfactant mixture was prepared using AOS and betaine surfactants, but without the addition of a nanofluid component (Formulation 1). The control surfactant formulation included a mixture of AOS and betaine at about a 4 to 1 ratio.

For the preferred embodiments, the AOS and betaine surfactant mixture (formulation #1) was combined with a nanofluid component in varying amounts (Formulations #2, #3 and #4). The nanofluid component included an emulsifying surfactant, a solvent, a co-solvent and water. The control and test formulations are identified in Table 1 below:

TABLE 1 Test Formulations Formulation 1 2 3 4 AOS 75.0%   66.4% 57.6% 49.6% Betaine 25.0%   16.6% 14.4% 12.4% Emulsifying surf 0% 6.1% 10.1% 13.7% Solvent 0% 3.7% 6.2% 8.4% Cosolvent 0% 5.1% 8.4% 11.4% Water 0% 2.0% 3.4% 4.6% Total 100.0%    100.0% 100.0% 100.0%

Formulations 2-4 represent examples of the preferred embodiments in which the AOS and Betaine are combined with increasing amounts of nanofluid ranging from 16.9% by weight (formulation #2) to 38.1% by weight (formulation #4). The AOS and betaine were mixed in a 4:1 weight ratio in each case.

The stability of the surfactant formulations was evaluated in brines to determine the resilience of the surfactant formulation under varying salinity (15% to 25%) and varying temperature (room temperature and 150° F.). The results of this study are presented in Table 2 below:

TABLE 2 Surfactant Solution phase behaviors at room temperature and 150° F. at varying salinity Surfactant Concentration = 5 gpt 15% 20% 22.5% 25% Formu- ME Salinity Salinity Salinity Salinity lation W % RT 150° F. RT 150° F. RT 50° F. RT 150° F. #1 0 CS CS CS CS TP TP TP TP #2 16.9 CS CS CS CS LH TP LH TP #3 28.1 CS CS CS CS LH LH LH TP #4 38.1 CS CS CS CS LH LH LH TP (CS—clear single phase; TP—two phase; LH—light hazy single phase)

Phase stabilization tests have determined that the ratio of AOS to betaine in the control formulation (#1) provides the best tolerance to salinity. When used at typical or expected injection concentrations (e.g., 5 gallons per thousand gallons=gpt), this surfactant solution remains clear in up to 20% salinity at both room temperature (RT) and reservoir temperature (150° F.).

As noted in Table 2, the preferred embodiments of the surfactant formulation exhibit increased resistance to salinity over the control formulation. If 16.9 wt % nanofluid is added, the surfactant solution becomes light hazy at room temperature up to 25% salinity and phase separation is apparent at 150° F. The surfactant solution maintains a light hazy but homogenous single phase at up to 22.5% salinity with increasing nanofluid concentration to 28.1 wt % and 38.1 wt %. These solutions remained stable at 150° F. for more than 4 weeks.

The increased stability achieved through the addition of the nanofluid is believed to be the result of a reduction of the critical micelle concentration. Betaine has a lower critical micelle concentration than AOS. The critical micelle concentration of a mixture of AOS and betaine follows the mixture rule. Adding the nanofluid to the betaine/AOS mixtures further reduces the critical micelle concentration, which makes the formulation more efficient and stable.

In addition to an increased resistance to salinity, laboratory tests have also confirmed that the preferred surfactant formulations exhibit lower formation retention rates than isolated betaine. FIG. 1 presents a comparison of the retention/adsorption (mg/m2) of betaine against the surfactant formulations in 15% salinity at room temperature.

It has been found that isolated betaine surfactant shows a much higher retention in the subterranean formation than AOS and the mixture of betaine and AOS (formulation 1), and adding the nanofluid into the betaine and AOS surfactant mixture (formulations 2-4) further reduces the betaine adsorption. Formulation #2 is a complex system that includes anionic, amphoteric, nonionic surfactants and solvents. It is believed that the dispersed nanofluid complex provides an oil/water interface that is occupied by the various surfactants thus minimizing the free energy between the betaine and the formation and thus decreasing retention. This reduced retention of betaine helps generate foam faster and decreases the amount of betaine needed for successful CO₂ diversion.

Having established preferable retention and salinity properties, the performance of the novel surfactant formulations was evaluated using a series of four dual-core oil recovery tests. For the dual-core oil recovery tests, a pair of clean, dry cores was inserted into core holders. The cores were selected to have different permeabilities. The confining pressure was added to prevent any bypass. Each core was purged by CO₂ gas for 60 minutes in order to eliminate any air inside, then the core was saturated using formation brine (FB) and the absolute permeability was measured. Crude oil was then injected from the top of the core until no further water came out. The oil saturated core was aged for 15 hours at 150° F. CO₂ was co-injected at a certain ratio with FB or treatment chemicals at 10 ft/day. The same flow velocity (10 ft/day) was used for all of the tests.

Secondary production (from bottom to top) flooding was performed with brine to a residual oil saturation, followed by a CO₂-Formation Brine (FB) co-injection as tertiary flooding. The CO₂-surfactant solution was then co-injected and the apparent viscosity of CO₂ foam in the presence of oil was measured. The volume of oil recovered in different stages was recorded.

In a first control test, 2 gpt of betaine in 15% salinity was used as the treatment product. The flooding by FB alone and CO₂-FB (1:1) co-injection recovered 20.0% and 19.5% oil, respectively. CO₂ diversion flooding recovered an additional 12.5% oil. The use of betaine alone produced a total oil recovery of 52.0%.

In a second control test, 5 gpt of formulation #1 (AOS to Betaine ratio of 4:1 surfactant mixture) in 15% salinity was used as treatment. The flooding by FB alone and CO₂-FB (1:1) co-injection recovered 38.3% and 16.1% oil, respectively. CO₂ diversion flooding recovered an additional 24.3% oil. Using a first preferred embodiment of the inventive surfactant formulation produced a total oil recovery of 78.7%.

In a third test, 5 gpt of formulation #2 (betaine, AOS and nanofluid) in 15% salinity was used as the treatment surfactant formulation. The flooding by FB alone and CO₂-FB (1:1) co-injection recovered 27.0% and 30.0% oil, respectively. CO₂ diversion flooding recovered an additional 25.1% oil. Using a second preferred embodiment of the novel surfactant formulation produced a total oil recovery of 82.1%.

For the final test, 5 gpt of formulation #2 (betaine, AOS and nanofluid) in 15% salinity was used as the treatment surfactant formulation. The flooding by FB alone and CO₂-FB (1:1) co-injection recovered 41.2% and 26.1% oil, respectively. CO₂ diversion flooding recovered an additional 22.2% oil for a total oil recovery of 89.5%.

Tests 1, 2 and 3 were conducted under immiscible conditions. CO₂ is partially miscible with the crude oil and thus only a fraction of the oil is capable of being recovered. Test 4 was performed under miscible conditions.

It is believed that the flooding by formation brine (FB) alone and CO₂-FB co-injection for Test 1 is lower because the CO₂ foam was generated slowly due to the high adsorption of betaine and resulting low betaine concentration. The oil recovery from FB and CO₂-FB co-injection for Test 2 and Test 3 is similar because they were conducted under the same flooding conditions, such as back pressure, salinity, temperature, and core permeability. Notably, however, the oil recovery rate during the CO₂ foam flooding is different between these tests. It was observed that the nanofluid surfactant formulation used in Test 3 recovered oil within 2.5 pore volume (PV), whereas the surfactant formulation in test 2 took about 6 PV to get to the plateau. These results prove that the nanofluid formulation greatly enhanced the CO₂ foamer properties and thus improved the oil recovery rate.

During use, it is currently contemplated that the inventive surfactant formulations may be used in concentrations ranging from 2-50 gallons-per-thousand gallons of carrier fluid, but more concentrated or dilute applications are contemplated as within the scope of the present invention and may be necessary depending on the wellbore conditions and treatment parameters. Particularly preferred concentrations are between 2-5 gallons-per-thousand gallons of carrier fluid.

Although preferred embodiments have been disclosed in the context of CO₂ flooding, it will be appreciated that the novel surfactant formulations may also be used with other foam-inducing gases, including nitrogen, hydrocarbons, hydrocarbon/solvent, hydrogen sulfide and flue gases or a combination thereof. Additionally, the novel surfactant formulations may also be applied in water flooding and foam diversion applications.

It is clear that the present invention is well adapted to carry out its objectives and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments of the invention have been described in varying detail for purposes of disclosure, it will be understood that numerous changes may be made which will readily suggest themselves to those skilled in the art and which are encompassed within the spirit of the invention disclosed and as defined in the appended claims. For example, surfactant and surfactant mixture selections can be modified and changed to take into account varying reservoir conditions. 

It is claimed:
 1. A foam generating surfactant formulation comprising: a betaine; an alpha-olefin sulfonate; and a nanofluid.
 2. The foam generating surfactant formulation of claim 1, wherein the betaine is selected from the group consisting of cocamidopropyl betaine and laurel betaine.
 3. The foam generating surfactant formulation of claim 2, wherein the alpha-olefin sulfonate is an anionic surfactant having between 8 and 18 carbon atoms per molecule.
 4. The foam generating surfactant formulation of claim 3, wherein the alpha-olefin sulfonate has between 8 and 12 carbon atoms.
 5. The foam generating surfactant formulation of claim 1, wherein the formulation includes about 1 part betaine to between about 2 and 5 parts alpha-olefin sulfonate.
 6. The foam generating surfactant formulation of claim 5, wherein the formulation includes about 1 part betaine to about 3.4 parts alpha-olefin sulfonate.
 7. The foam generating surfactant formulation of claim 1, wherein the nanofluid comprises an oil-in-water nanofluid.
 8. The foam generating surfactant formulation of claim 1, wherein the nanofluid comprises: an emulsifying surfactant; a solvent, a co-solvent; and water.
 9. The foam generating surfactant formulation of claim 8, wherein the solvent is a citrus terpenes.
 10. The foam generating surfactant formulation of claim 9, wherein the solvent is d-limonene.
 11. The foam generating surfactant formulation of claim 8, wherein the emulsifying surfactant comprises a mixture of ethoxylated castor oils and ethoxylated alcohols.
 12. The foam generating surfactant formulation of claim 11, wherein the emulsifying surfactant has a hydrophile-lipophile balance value of between 8 and
 18. 13. The foam generating formulation of claim 8, wherein the emulsifying surfactant is selected from the group consisting of ethoxylated alcohol and ethoxylated vegetable oil.
 14. The foam generating surfactant formulation of claim 1, wherein the foam generating surfactant formulation comprises: between about 5 and about 70 percent by weight alpha-olefin sulfonate; between about 10 and about 20 percent by weight betaine; between about 5 and about 15 percent by weight emulsifying surfactant; between about 3 and about 10 percent by weight solvent, between about 3 and about 15 percent by weight co-solvent; and between about 0 and about 10 percent by weight water.
 15. A method for producing a treatment foam in a well, the method comprising the steps of: mixing together a betaine, an alpha-olefin sulfonate and a nanofluid to form a foam generating surfactant formulation; pumping the foam generating surfactant formulation into the well; and forcing gas into the well to contact the foam generating surfactant formulation to produce the treatment foam.
 16. The method of claim 15, wherein the step of pumping the foam generating surfactant formulation further comprises adding the foam generating surfactant formulation to a carrier fluid that is pumped into the well.
 17. The method of claim 16, wherein the foam generating surfactant formulation is added to the carrier fluid in a ration of between about 2 and 5 gallons per thousand gallons of carrier fluid.
 18. The method of claim 15, wherein the step of forcing gas into the well further comprises forcing into the well a gas selected from the group consisting of nitrogen, hydrocarbon gases, hydrocarbon/solvent gas mixtures, hydrogen sulfide and flue gases or combinations thereof. 